1. Field of the Invention
The present invention relates to an optical system using an optical material having extraordinary partial dispersion, and more particularly, to an optical system suitable for use in a photographing optical system for a silver-halide film camera, a digital still camera, or a video camera, or to an optical system suitable for use in a projection optical system for a liquid crystal projector.
2. Related Background Art
With an increase in high resolution of a digital still camera, it is necessary for a photographing lens for image formation in the digital still camera to sufficiently correct color blurring in a white light or aberration related to chromatic aberration which influences the resolution of an image, in addition to the correction of aberration related to image performance in monochrome, such as spherical aberration or comatic aberration.
In general, in order to expand a photographing region, a zoom lens having a higher zoom ratio or a lens for obtaining a super wide-angle view field or a super telephoto view field is strongly desired as the photographing lens. However, in particular, when a focal length of the photographing lens is increased to improve telephotographing, it becomes an important factor to correct chromatic aberration (axial chromatic aberration) with respect to not only a primary spectrum but also a secondary spectrum in order to obtain high quality image performance.
As regards the super wide-angle photographing lens, it is necessary to obtain a strong retrofocal action, in particular, in the case where a single-lens reflex camera which requires a back focus longer than the focal length is used or the case where a digital still camera having a solid-state image pickup element which requires a telecentric characteristic in an image side is used. In the photographing lens having the strong retrofocal action, symmetry is lost, with the result that large chromatic aberration of magnification is caused. Therefore, it is important to correct such chromatic aberration of magnification.
Up to now, it has been known that a lens made of extraordinary dispersion glass is disposed in an optical system in order to correct a secondary spectrum of the chromatic aberration.
Examples in which glass having an extraordinary dispersion property is used for a zoom lens are described in Japanese Patent No. 3097399, Japanese Patent Application Laid-Open Nos. 2002-062478 (corresponding to U.S. Pat. No. 6,594,087), H08-248317, and 2001-194590 (corresponding to U.S. Pat. No. 6,404,561). The zoom lens includes four lens units, that is, a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power, in order from an object side.
An example in which a diffractive optical element having a chromatic aberration correcting effect better than that of an extraordinary dispersion glass material is used for a zoom lens is described in Japanese Patent Application Laid-Open No. H09-211329 (corresponding to U.S. Pat. No. 5,872,658).
A liquid material which is relatively high dispersion and indicates a relatively extraordinary partial dispersion property has been known as a material having a chromatic aberration correcting action similar to that of a diffractive optical element. An apochromatic optical system that uses the liquid material has been proposed in U.S. Pat. No. 4,913,535.
In order to improve diffractive efficiency, it has been proposed in Japanese Patent Application Laid-Open No. 2001-074901 (corresponding to EP 1 065 531 A3) to use a replica material, obtained by mixing ITO fine particles into a resin, for a diffractive grating.
An example in which an aspherical surface made of a resin layer is provided in a first lens unit to correct monochromatic aberration has been known in Japanese Patent Application Laid-Open No. 2001-021803.
When the diffractive optical element is used for the zoom lens, the chromatic aberration correcting effect larger than that of the extraordinary dispersion glass material is obtained. However, the diffractive efficiency of the diffractive grating depends on an incident angle of a light beam. Therefore, when the zoom lens is used in a condition in which an incident condition of the light beam changes during zooming, an installation position of the diffractive optical element may be limited or the diffractive efficiency according to a zoom position may be reduced.
A method of forming an ultraviolet curing resin or the like using a metal mold has been known as a method of manufacturing the diffractive optical element. However, the diffraction optical element has extremely high manufacturing sensitivity with respect to the diffraction efficiency, so very high mold precision and very high formation precision are required. Therefore, there is also a problem in that a manufacturing cost is high at the current moment.
The material disclosed in U.S. Pat. No. 4,913,535 is a liquid, so a structure for sealing the liquid is required and cannot be easily manufactured. There is also a problem with respect to changes in characteristics such as a refractive power and a dispersion characteristic, which are caused by a variation in temperature, so an environmental resistance is insufficient. Further, it is hard to obtain a sufficient chromatic aberration correcting action because the Abbe number is relatively large, an extraordinary partial dispersion property is relatively small, and an interface with air is not obtained.
When an aspherical surface is made of a normal resin material disclosed in Japanese Patent Application Laid-Open No. 2001-074901 (corresponding to EP 1 065 531 A3), the chromatic aberration correcting effect cannot be obtained.